1. Field of the Invention
This invention relates generally to the field of oxides of metals and non-metals and, more particularly, to processes for the preparation of such oxide materials which are water-free, and for the formation of optical components from such water-free oxide materials, which transmit radiation in the near infrared wavelength region.
2. Description of the Prior Art
Metal and non-metal oxides and particularly oxide glasses, such as silicates, phosphates, and borates, are frequently used to form optical components, such as windows and lenses, in optical systems requiring the transmission of radiation of a particular wavelength. However, it is known that polyatomic impurities degrade the optical transparency of the host material in the near-infrared radiation range (i.e. 0.75 to 2.5 micrometers). This effect has been studied in detail with regard to alkali halide crystals, as discussed by C. J. Duthler in the publication entitled "Extrinsic absorption in 10.6-.mu.m-laser-window materials due to molecular-ion impurities," in the Journal of Applied Physics, Vol. 45, 1974, pages 2668-2671, and in U.S. Pat. No. 3,932,597, assigned to the present assignee. In addition, this degraded optical transparency in the near-infrared range has been observed in oxide materials as well. In particular, water vapor poses a difficult problem because this species is an ubiquitous impurity that is often uncontrolled in all phases of materials preparation and processing. The impurities derived from water, namely, hydroxyl ions and hydrogen ions, degrade the near-infrared transmission of oxides, as discussed, for example, in the book entitled "Ultrapurity Methods and Techniques", edited by M. Zief and R. Speights, Marcel Dekker Inc., 1972, at pages 330-331, and the publication entitled "Water Content and Infrared Transmission of Simple Glasses" by Anna J. Harrison, in the Journal of the American Ceramic Society, Vol. 30, No. 12, 1947, pages 362-366.
Conventional methods for preparing metal oxides yield a product which has a substantial concentration of hydrogen impurities. For example, an oxysalt, such as a sulfate, nitrate, or carbonate may be prepared by the reaction of calcium hydroxide and sulfuric, nitric, or carbonic acid, respectively, in a water-rich medium, and the oxysalt may then be calcined (i.e. heated under oxidizing conditions). For example, calcium carbonate is calcined to calcium oxide and carbon dioxide. In another known process for forming metal oxides, the free metal may be combusted in air, which invariably contains water. In still another known process, a metal oxide may be formed by hydrolysis of a metal halide, followed by calcination to the oxide. These preparative methods discussed above are well-known and are described in the book entitled "Treatise in Inorganic Chemistry", by H. Remy, translated by J. S. Anderson and edited by J. Kleinberg, Elsevier, 1956. In all of the above-described processes, the oxide is exposed to an environment containing water, which becomes incorporated in the oxide. Even when these oxides are calcined, the oxide is exposed to water in the surroundings, such as water resulting from continuous outgassing from the walls of the apparatus used.
It is the alleviation of this prior art problem of the degraded optical transparency of oxides in the near infrared range due to water impurities to which the present invention is directed.
In the particular case of fused silica, (i.e. silicon dioxide in the amorphous or vitreous state), there have been attempts to solve this prior art problem mentioned above by forming a fused silica material with negligible free-hydroxyl content. For example, the vapor-phase hydrolysis or oxidation of silicon compounds in a flame yields 0.12 weight percent free-hydroxyl in silica. In addition, flame fusion of a quartz crystal yields 0.04 weight percent free-hydroxyl, while electrical fusion of quartz crystal powder yields 0.0003 weight percent free-hydroxyl. The values listed above are presented in the article entitled "Water in vitreous silica: Part I. Influence of water content on the properties of vitreous silica", by G. Hetherington and K. H. Jack, in Physics and Chemistry of Glasses, Vo. 3, No. 4, 1962, pages 129-133. However, the latter two processes use quartz which is usually from a natural source and therefore of limited supply and relatively costly. In addition, these approaches for forming fused silica as described above are forced to resort to energy-intensive processes requiring relatively high temperatures and long dwell times, because of the slowness of the rate by which the hydroxyl species are eliminated by the outgas of water vapor.
The present invention is further directed to providing a fused silica material which is free of water impurities by a process which is free from reliance on costly starting materials and high energy inputs as are required in the prior art processes discussed immediately above.